1. Field of the Invention
The present invention relates generally to solar cells and, more particularly, to a solar cell of improved conversion efficiency and formed with a plurality of grooves on its surfaces where the direction of the front grooves is at an angle to the direction of the back grooves.
2. The Prior Art
Solar cells are pn-junction devices which convert incident light energy directly into electrical energy. Solar cells are widely known but not yet widely used. Their use has been restricted by competitive economics, specifically their lack of ability to compete in the marketplace with other known sources of electrical energy. At the heart of this is the relatively low conversion efficiencies of most solar cells, typically in the 10-14% range, with only the best and the most expensive solar cells reaching efficiencies of about 17-18%.
As known, for solar cells with low surface recombination at their back surfaces, the open circuit voltage (V.sub.oc), hence efficiency, increases as the wafer thickness decreases. As the solar cell is made thinner, however, the short circuit current (I.sub.sc) decreases. This is so because not all of the incident light striking the solar cell is absorbed in the reduced optical path length (L) between the front and the back surfaces. In order to maintain light absorption at optimum level without at the same time increasing the thickness of the cell, it is desirable therefore to increase the optical path length (L) of the light traveling within the solar cell.
The desirability of increasing the optical path length (L) in solar cells has been recognized for some time. In a simple cell, light simply passes through the cell once, hence its optical path length (L) equals the cell's thickness, i.e., L=1. By adding a back surface reflector (BSR) to the back surface of the cell, its optical path length is doubled (the light passing through twice the cell), i.e., L=2. By also texturing the front surface of the cell, together with the BSR, the optical path length is increased to about 2.5 times the thickness of the cell, i.e., L=2.5. On silicon cells, texturization is usually accomplished with an anisotropic etching medium, such as an alkali hydroxide solution, often sodium hydroxide or potassium hydroxide solution. Rather than texturing the front surface, the back surface may be textured. The angle of back texturization must be greater than .theta./2, where .theta.=arc sin (n.sub.2 /n.sub.1), and n.sub.2 is the index of refraction of the solar cell material and n.sub.1 is the index of refraction of the external medium, such as air, where n.sub.1 equals about one. For a silicon solar cell, n.sub.2 is about 3.7, varying slightly with wavelength. Light entering through the front surface first is reflected back at an angle by the textured back surface toward the front surface. If the angle of incidence of light striking the inside front surface is greater than .theta., it will be reflected back again at an angle toward the textured back surface. The textured back surface then reflects the light so that it now exits through the front surface. The light thus makes two passes in each direction, two of those being at an angle, before exiting from the cell. The resultant optical path length is about 4.6, i.e., L=4.6.
Attempts at texturing both the front and the back surfaces, however, tended to shorten rather than lengthen this optical path length. This is due to the fact that light entering through one facet in the front surface most likely will reflect from a facet parallel to the first facet, resulting in only one light pass in each direction. Thus, the optical path length is about the same as that of the front textured cell, i.e., L=2.5.
Some workers in the art have suggested more sophisticated back texturing, namely texturing at very low angles. See D. Redfield, Applied Physics Letters, Vol. 25, No. 11 (1 Dec. 1974), pp. 647-648; and M. B. Spitzer et al., Proceedings 13th IEEE Phtovoltaic Specialists Conference (1980), p. 375. In this case, the optical path length depends on the exact sequence of reflections taken by the incident light, which is a function of the position of the entering light ray. For the most probable sequence of reflections, four passes throught the solar cell are made in each direction, resulting in an optical path length of about 11.4 times the thickness of the cell, i.e., L=11.4. Such an optical path length enhancement, however, requires a flat front surface, which has a higher reflectivity than a textured surface. Furthermore, if the back surface low angle texturing is produced by anisotropic etching, the process requires the employment of an unusual, hence costly, material orientation.
Other workers in the art (see E. Yablonovitch and G. D. Cody, IEEE Transactions on Electron Devices, Vol. ED-29, No. 2, February 1982, pp. 300-305) have investigated optical path length increase in solar cells by employing arbitarily rough back surfaces. With the use of arbitarily rough back surfaces, the direction of the light rays is assumed to be completely randomized by each reflection. Theoretically at least, such complete randomization results in a maximum achievable total optical path length enhancement of 4(n.sub.2 /n.sub.1).sup.2, where (n.sub.2 /n.sub.1).sup.2 represents enhancement due to total internal reflection at the cell's front surface, and times 2 due to total reflection by a BSR and the other times 2 due to the obliqueness of the angles of the light rays traversing the body of the solar cell. However, this theoretical maximum enhancement is achievable only if both cell surfaces are totally random in light reflection and if the randomness on the opposing surfaces are not correlated. The first of these can at best be approximated and the second improved, as for instance, by sandblasting, see U.S. Pat. No. 3,487,223 issued to A. E. St. John. When using sandblasting on the back surfaces of solar cells, damage to the surfaces occasioned by sandblasting can adversely affect cell performance.